During a joint replacement procedure, it is necessary to manipulate prosthesis components into their required positions with respect to patient anatomy. In the case for example of a hip replacement prosthesis, it is necessary to manoeuvre an acetabular cup into a prepared patient acetabulum, and it may also be necessary to impact the cup into the prepared recess. It is known to use specific tools for the manipulation and impaction of prosthesis components in this manner.
In order to allow a gripping or impaction tool to gain purchase on an implant, it is usual for the implant to include one or more attachment features onto which the gripping and/or impaction tool can engage in order to manipulate the implant. This is particularly the case with acetabular cups, which do not otherwise lend themselves to easy engagement with a gripping or impaction tool. Typical attachment features for an acetabular cup might include a threaded apical bore, operable to receive a threaded rod, as well as projecting lugs and undercuts, all designed to engage with corresponding attachment features on a gripping and/or impaction instrument. This arrangement works well for multiple body acetabular cups, of kind to comprise an external shell and separate internal bearing component. In this type of construction, the external shell may be provided with any appropriate attachment features for interaction with a gripping or impaction tool. Once in place, these attachment features are completely covered by the separate bearing component that is fitted into the outer shell.
Attachment to a gripping or impaction tool becomes more problematic when considering single body, or so called monobloc acetabular cups. This type of implant component comprises a single body having an outer bone engaging surface and an inner bearing surface. In such a component, the capacity to accommodate additional attachment features is severely limited. The internal bearing surface must remain free of any surface discontinuities or other aberrations and thus is not available for the provision of attachment features. Even temporary attachment mechanisms have proven extremely difficult to implement. The condition of the bearing surface is vital to the success and longevity of the implant. Thus, any temporary attachment procedure must be carefully controlled to ensure that no possibility of damage to the highly polished bearing surface can arise. Similar issues are also encountered in the assembly of a multiple body acetabular cup. As noted above, the shell component of a modular acetabular prosthesis may comprise attachment features, as these will be covered once the prosthesis is assembled. However, the bearing component must also be gripped and manipulated into place on the shell, and, as in the case of a monoblock cup, the internal bearing surface of the bearing component must remain smooth and the risk of damage to the surface must be minimised.
An additional issue that must be considered within the context of implant manipulation is the release of the implant from the relevant tool once the implant is in position. The implant may be considerably less accessible once in position, and removal or release form a tool may involve gaining direct access to the implant through the wound incision. Such direct contact is undesirable and carries risks of additional accidental contact with the implant and consequent damage to the bearing surface.